Quantifying old-growthness of lowland European beech forests by a multivariate indicator for forest structure

Abstract

Quantifying the degree of old-growthness of forests is reasonable to assess their conservation value and guide management decisions. This study aimed at developing and applying an indicator to quantify the old-growthness of forest structure on potential beech forest sites in Central Europe which exhibit a long history of forest management. A set of structural variables was derived from sample plot inventories in European primeval beech forests in Eastern Slovakia (classified as old-growth) and 39 comparison stands of different management intensity, age and tree species composition in the North German lowlands. The comparison stands were arranged in triplets, consisting of i) a > 100 year old European beech forest left to natural development (ND), ii) 80 to 100 year old Scots pine forest with a certain amount of deciduous trees in the understory (OP), and iii) <80 year old Scots pine forest (YP).The initial number of 134 attributes was condensed to 27 variables representing ten thematic groups. Selection criteria were, i) typical for the old-growth state, ii) widespread and meaningful, and iii) exhibiting no multicollinearity within the respective group. The developed old-growth indicator (OGI) measures the degree of overlap of the 5th–95th percentile ranges of a certain comparison stand with the primeval forests. OGI achieves values between 0 and 1 and allows the consideration of all thematic groups and variables separately as well as calculating an aggregated value.We derived plausible OGI-values of between 0.71 and 0.74 for the primeval forests, 0.13 to 0.42 for the ND stands, and 0.07 to 0.30 for the OP and 0.03 to 0.26 for the YP stands. We postulated that OGI provides a comprehensive and reproducible indicator of the maturity of a forest stand on an empirical basis that allows for differentiated, as well as easy to handle, aggregated evaluations. An additional advantage is the implementation into the established workflow for forest surveys of national parks and strict forest reserves in Germany.